An electrically powered vehicle utilizing a motor to generate vehicle driving force, such as an electric vehicle, a hybrid vehicle, or a fuel cell vehicle, has a secondary battery (hereinafter, also simply referred to as “battery”). The secondary battery stores electric power to drive the motor. In the electrically powered vehicle, during reduction of speed, regenerative braking force is generated by the motor, thereby providing regenerative power generation to charge the secondary battery with the resulting electric power.
Thus, the secondary battery mounted on the electrically powered vehicle is repeatedly charged and discharged during vehicle traveling. Accordingly, during the vehicle traveling, it is important to control an amount of charge stored in the secondary battery. As an amount of state representing the amount of stored charge, SOC (State Of Charge) has been used conventionally. The SOC is represented by a ratio of the amount of stored charge at present to the amount of stored charge in the fully charged state. In the fully charged state, SOC=100(%). In a state in which the secondary battery is not changed at all, SOC=0(%).
Normally, with SOC=approximately 50-60(%) being a control objective, the secondary battery is controlled to be charged and discharged during the traveling such that the SOC falls within a range of a control lower limit value (for example, 30(%)) to a control upper limit value (for example, 80(%)). Specifically, when the SOC comes close to the control lower limit value, the vehicle is controlled to urge charging of the secondary battery. As one example, in a hybrid vehicle, a power generator is operated using an output of an internal combustion engine, thereby generating electric power for charging the secondary battery. On the other hand, when the SOC comes close to the control upper limit value, the vehicle is controlled to limit or prohibit regenerative braking.
Meanwhile, in the electrically powered vehicle, a battery pack is generally used to obtain a high output voltage. The battery pack has a multiplicity of battery cells connected to one another in series. However, in such a battery pack, temperature is not uniform among the cells. Particularly in an operating environment such as that in a vehicle, a temperature difference is relatively likely to take place between the cells. As a result, charging efficiency and full charging capacity differ among the cells, which leads to variation of actual SOCs among the cells constituting the battery pack.
In view of this, Japanese Patent No. 3533076 (PTL 1) describes a technique for precisely detecting a state of charge stored in the entire battery pack even when the SOC variation becomes large among the cells constituting the battery pack.
In Japanese Patent No. 3533076 (PTL 1), the battery pack is divided into a plurality of battery blocks. Variation in the amounts of stored charge among the battery blocks is detected. Then, based on a value of the detected variation and the upper and lower limit values of the amounts of stored charge in the battery blocks, a movable range for the amounts of stored charge is determined. Then, an amount (N-SOC) of stored charge in the entire battery pack is defined in accordance with a positional relation of the state of charge stored in each of the plurality of battery blocks at present relative to this movable range.